EMDR generally works well in assisting people with the kinds of reexperiencing of trauma that you are describing. I can't comment specifically on your case, because I haven't had the opportunity to evaluate you, so let me just make some general comments.

When EMDR clients have both a recent traumatic experience and a history of prior traumas, it is possible that the resolution of the recent experience may require resolution of the prior experiences as well. The EMDR practitioner in such cases needs to do preliminary screenings to make sure it is clear which protocol to use.

The scary part you refer to is a common feeling, because in a way during the EMDR the person "reexperiences" the trauma, which can't be too great, right? However, clients need to remember that it isn't really happening now, and just to watch and notice the trauma as is goes by, as if watching it through a train window or on a movie screen. The therapist helps remind the client that it isn't happening now, that they are safely sitting in an office, and that its okay to just notice the memory of the trauma. The therapist also helps get it unstuck if it starts to get stuck, and to help close it down at the end of the session, so the person feels comfortable after. After this session (sometimes more than one is needed), typically the client feels much much better, the nightmares and pictures stop, and it is a great relief for the client.

If a person has a complicated trauma history, certainly many sessions are likely to be needed to get the relief.

I hope this gives the information you need in preparation for your EMDR.